1. Field of the Invention
The present invention relates to injection stretch blow molding process and apparatus which can continuously make a cycle of molding steps by moving a neck mold while it holds a preform, and particularly to an improved system for conveying the neck mold.
2. Description of the Related Art
Systems for moving the neck mold through a plurality of stages realizing a cycle of molding steps are disclosed in Japanese Patent Publication No. 22096/1978 and Japanese Laid-Open Patent Application No. 40316/1989.
Patent Publication 22096/1978 discloses a system comprising a circular rotary member and four neck molds on the rotary member. The four neck molds are disposed opposite to the respective one of four stages, that is, injection molding stage, heating stage, stretch blow molding stage and ejection stage, all of which are located around the rotary member. When the rotary member is intermittently rotated, each of the neck molds will be sequentially moved through the four stages.
Laid-Open Patent Application 40316/1989 discloses a system similar to the system disclosed in Patent Publication 22096/1978, except that it comprises a first path along which each of the neck molds is conveyed sequentially through the respective one of various stages for realizing the injection stretch blow molding process, and a second path formed parallel to the first path and used to return each of the neck molds from the final molding stage in one cycle to the first path. The neck molds are conveyed through the two paths by conveyor means which comprises an actuator for slidably moving each of the neck molds from one stage to the next stage, means for transferring the neck mold between the first and second paths and another actuator for returning the neck mold from the second path back to the first path.
The two prior art techniques are common to each other in that all the neck molds are circulated through a loop defined by the first and second paths. However, these prior art techniques raise the following problems when they are applied directly to an injection stretch blow molding process of forming preforms with increased bore diameters.
In order to mold preforms having increased bore diameters, increasing the bore diameter of the neck molds used therein and also increasing a distance between each adjacent neck molds longitudinally spaced apart from each other cannot be avoided when a plurality of preforms are simultaneously format one stage. Therefore, increasing span between tie rods used to guide a clamping mechanism in the vertical movement thereof cannot be avoided.
In the construction disclosed in the Patent Publication 22096/1978, the tie rods must be located at such a position that they will not interfere with various parts rotated by the rotary member. If the span between the tie rods is increased for the above purpose, the use of a clamping mechanism having a clamping force equal to several hundreds tons for molding performs having an increased bore diameter must be designed to have a sufficient strength to resist a bending moment produced therein. This results in increase of the size of the entire molding system.
On the other hand, the construction disclosed in Laid-Open Patent Application 40316/1989 can maintain a minimum span between a minimum tie rods even if the bore diameter in preforms to be molded is increased, because each of the neck molds is slidably moved at an injection molding stage. However, such a construction must separately have the additional or second path for returning the neck molds back to their original position, other than the first path for conveying the neck molds through the injection stretch blow molding process. This also results in increase of the size of the entire molding system.